This invention concerns underdrain filtration systems for water purification or sewage treatment systems which employ a filtering medium of stacked granular material retained above a tank floor by perforated metal underdrain structures. More particularly, the invention is concerned with an improvement whereby the perforations in the metal underdrain structures supporting the granular material are formed in an efficient manner so as to establish a high rate of consistency among the opening sizes, as well as promoting a better fluid flow pattern during backflushing.
An example of an underdrain filtration system is shown in application Ser. No. 09/170,870, U.S. Pat. No. 6,090,284. As pointed out in that application and patent, such granular filtration systems typically have corrugated or undulating-shaped perforated underdrain structures, and the systems are equipped to backwash the granular material covering the underdrain units by water, and usually by air from an adjacent source. The assignee""s U.S. Pat. No. 4,331,542 also describes an underdrain structure for use with such gravity-fed granular filtration systems, fitted with water backflushing and air scouring.
As explained in those patents, the subject high rate gravity filters usually have filter beds with gravel of varying sizes. Liquid from the tank or vessel flows downwardly by gravity through the filter medium and exits the filtration tank through openings in the underdrain structures that support the granular filtration medium. Typically the coarsest gravel of the medium is at the bottom, against the underdrain structure, with the gravel decreasing in size up to the finest gravel or sand, located at the top of the filter bed. Some filter beds are all sand, and the bed may be covered with finely granulated anthracite coal or other types of filter material.
Backwashing and air scouring are important aspects of these types of gravity filters, for cleaning the filters against clogging. Water or a combination of air (or gas) and water is passed up through the filter bed, in the opposite direction of normal filtration. Air may be from a tube adjacent to the underdrain structures. Uniform distribution of the backwashing fluids is a goal of most underdrain structures. This includes the systems described in the above-referenced patents, as well as some of the underdrain systems of the following patents: U.S. Pat. Nos. 4,659,462, 4,707,257, 5,015,383, 5,019,259, 5,156,738, 5,160,614, 5,269,920, 5,332,497, 5,462,664, 5,489,388, 5,512,174, 5,639,384.
Application Ser. No. 09/170,870, U.S. Pat. No. 6,090,284, shows an embodiment of an underdrain structure wherein the water-passing perforations are in the form of narrow slits, small enough to support a bed of sand without allowing the sand to pass through the openings. Slit openings were made by laser cutting, as opposed to an earlier practice of forming the openings as {fraction (3/16)}xe2x80x3 diameter holes, over which gravel was placed.
The consistency of the opening sizes is important for proper water distribution in backflushing and balancing of pressure drop across the entire underdrain structure. Laser cutting is fairly expensive and has typically not been capable of providing a highly consistent range of opening width. The openings formed by laser cuts in the experience of the applicant ranged from about 0.008xe2x80x3 to 0.012xe2x80x3, a variation of 0.002xe2x80x3 in both directions from the target size of 0.010xe2x80x3. The resulting differences in pressure drop tended to cause inefficiency in the action of the backwashing water in cleaning the surrounding filter medium. Moreover, even if these slots could be formed perfectly at the target width, the slots would not provide an optimum pattern of pressurized backwash water flow for filter cleaning, as does the system of the invention described below.
The invention is an improvement in the fabrication and performance of the filter media-retaining underdrain. Instead of round holes or laser-cut slots, the invention utilizes a machine punch process to produce fine openings in a metal plate of specific thickness gauge, resulting in openings of a very consistent width. In one embodiment the openings are 0.010xe2x80x3 in width and sufficiently small and consistent to directly retain a filter medium (sand) ranging from 0.3 mm to 0.5 mm. The punched plate preferably is then folded into a trapezoidal shape such as shown in the above-referenced U.S. Pat. Nos. 4,331,542 and 6,090,284. The trapezoidal underdrain structure defines a conduit for carrying filtered water or wash water in a municipal water treatment filter or tertiary treatment in a municipal waste water treatment filter.
The stampings preferably are circular in shape, although they could be other curved shapes or polygonal, and produce a plurality of slots which lie generally in a circuit defined by the space between a raised disc and the surrounding flat area of the underdrain material. In one preferred embodiment the raised discs are circular, with two to four bridges between ends of adjacent slits, these metal bridges being left remaining from the stamping of the discs. Two such bridges between two arcuate slots are generally sufficient, but three bridges will provide more stability of the aperture sizes in the event of rough handling or other forces experienced during transportation, installation and use.
A very highly consistent slit width can be formed in this way, varying less than 0.001xe2x80x3 in either direction, and thereby improving consistency of pressure drop throughout the underdrain. As a result of the small and consistent dimensions achievable with the process of the invention, underdrain structures can support the sand filter medium directly on the underdrain plates, thus eliminating the space normally required for progressively smaller gravel layers positioned over the underdrain to support the sand above, as in previous implementations.
A further benefit of the stamped-disc orifices is that the raised discs during backflushing through the underdrain create an outwardly diverging water flow pattern beneficial in clearing trapped particles from the filter medium in the vicinity of the underdrain apertures.
Tests were performed on underdrain sections formed according to the invention were tested for pressure loss. The pressure loss was found to be much lower than head loss through underdrain structures with laser-cut slots of the same total area of openings. This is due to the consistency of the openings and the lack of irregularity along the edges of the slots which are encountered with laser-cut slots.
It is thus among the objects of the invention to improve the structure, manufacturing procedure and cost of metal underdrains for granular filter beds, through use of a stamping process that forms raised discs in the metal structure, defining slits arranged generally in a circuit. These and other objects, advantages and features of the invention will be apparent from the following description of a preferred embodiment, considered along with the accompanying drawings.